Soybean, a legume, has become the world's primary source of seed oil and seed protein. In addition, its utilization is being expanded to the industrial, manufacturing and pharmaceutical sectors. Soybean productivity is a vital agricultural and economic consideration. Unfortunately, soybean is host to one of the widest ranges of infectious pathogens of all crops. More than a hundred different pathogens are known to affect soybean plants, some of which pose significant economic threats. Improving soybean disease tolerance to these many pathogens is crucial to preventing yield losses.
Charcoal Rot (or alternatively referred to herein as “Charcoal Rot Drought Complex”) is caused by the fungus Macrophomina phaseolina. The fungus has a particularly wide geographic distribution and is found throughout the world. M. phaseolina is most severe between 35° North and 35° South latitude (Wyllie, (1976) ‘Macrophomina phaseolina—Charcoal Rot’ P 482-484 In L. D. Hill (ed.) World Soybean Research Proc of the World Soybean Res. Conf., Champaign, Ill. Interstate, Danville, Ill.). The fungus also has a wide host range and infects over 500 crop and weed species and is highly variable. Known major crop hosts include alfalfa, maize, cotton, grain sorghum, peanut and soybean.
In localized areas, yield losses can be as high as 90%. In the period from 1996-2005, charcoal rot was the third leading cause of soybean yield loss in the U.S. Average annual losses were 29 MM bushels resulting in approximately $188 MM annual income loss. Only soybean cyst nematode and phythophthora root rot caused greater economic loss during that period (Wrather and Koenning (2006) ‘Soybean Disease Loss Estimates for the United States, 1996-2006’. University of Missouri—Columbia Agriculture Experiment Station. November 2006 http://aes.missouri.edu/delta/research/soyloss.stm Dec. 5, 2007).
Complete or vertical resistance to M. phaseolina has not been identified in soybean, which strongly suggests that a single gene conferring resistance does not exist. In most field and greenhouse evaluations, the great majority of soybean cultivars have been found to be either highly or moderately susceptible to M. phaseolina. Only a few cultivars have been identified as possessing partial or horizontal resistance (Smith and Carville (1997) ‘Field screening of commercial and experimental soybean cultivars for their reaction to Macrophomina phaseolina’ Plant Dis 81:804-809).
It is the goal of the plant breeder to select plants and enrich the plant population for individuals that have desired traits, for example, pathogen tolerance, leading ultimately to increased agricultural productivity. It has been recognized for quite some time that specific chromosomal loci (or intervals) can be mapped in an organism's genome that correlate with particular quantitative phenotypes. Such loci are termed quantitative trait loci, or QTL. The plant breeder can advantageously use molecular markers to identify desired individuals by identifying marker alleles that show a statistically significant probability of co-segregation with a desired phenotype (e.g., pathogenic infection tolerance), manifested as linkage disequilibrium. By identifying a molecular marker or clusters of molecular markers that co-segregate with a quantitative trait, the breeder is thus identifying a QTL. By identifying and selecting a marker allele (or desired alleles from multiple markers) that associates with the desired phenotype, the plant breeder is able to rapidly select a desired phenotype by selecting for the proper molecular marker allele (a process called marker-assisted selection, or MAS). The more molecular markers that are placed on the genetic map, the more potentially useful that map becomes for conducting MAS.
Despite significant advances in research directed towards improved crop tolerance to Charcoal Rot Drought Complex, there remains a need in the art for improved soybean strains that are tolerant to Charcoal Rot and its causative agents, namely Macrophomina phaseolina infection and low-available water growth conditions. There is a need in the art for methods that identify soybean plants or populations (germplasm) that display tolerance to Charcoal Rot Drought Complex. What is needed in the art is to identify molecular genetic markers that are linked to Charcoal Rot Drought Complex tolerance loci in order to facilitate MAS. Such markers can be used to select individual plants and plant populations that show favorable marker alleles in soybean populations and then employed to select the tolerant phenotype, or alternatively, be used to counterselect plants or plant populations that show a Charcoal Rot Drought Complex susceptibility phenotype. The present invention provides these and other advantages.